The present invention relates to a surface shape recognition apparatus and, more particularly, to a surface shape recognition apparatus for recognizing the small surface shape pattern of a human finger or animal nose.
In the social environment of today where the information-oriented society is developing, the security technology has taken a growing interest. For example, in the information-oriented society, a personal authentication technology for constructing an electronic money system is an important key. In fact, authentication technologies for implementing preventive measures against burglary and illicit use of cards are under active research and development (for example, Yoshimasa Shimizu, xe2x80x9cA Study on the Structure of a Smart Card with the Function to Verify the Holderxe2x80x9d, Technical Report of IEICE, OFS92xe2x80x9432, pp. 25-30, (1992)).
Such authentication techniques include various schemes using a fingerprint or voiceprint. Especially, many fingerprint authentication techniques have been developed. Fingerprint authentication schemes are roughly classified into optical read schemes and schemes of converting the three-dimensional pattern on the skin surface of a fingertip into an electrical signal using human electrical characteristics and outputting the electrical signal.
In an optical read scheme, a fingerprint is received as optical image data mainly using light reflection and a CCD images sensor and collated (Japanese Patent Laid-Open No. 61-221883).
Another scheme has also been developed, in which a piezoelectric thin film is used to read the pressure difference on a finger skin surface (Japanese Patent Laid-Open No. 5-61965). As a scheme of converting a change in electrical characteristics due to contact of skin into an electrical signal distribution to detect the shape of fingerprint, an authentication scheme of detecting a resistance or capacitance change amount using a pressure sensitive sheet has been proposed (Japanese Patent Laid-Open No. 7-168930).
However, of these techniques, the scheme using light is hard to achieve size reduction and versatility, and its application purpose is limited. The scheme of sensing the three-dimensional pattern at a fingertip can hardly be put into practical use and is poor in reliability because of special materials and difficulty in working.
A capacitive fingerprint sensor using an LSI manufacturing technology has also been proposed (Marco Tartagni and Roberto Guerrieri, A 390 dpi Live Fingerprint Imager Based on Feedback Capacitive Sensing Scheme, 1997 IEEE International Solid-State Circuits Conference, pp. 200-201 (1997)). In this method, small sensors two-dimensionally arrayed on an LSI chip detect the three-dimensional pattern of a skin using a feedback electrostatic capacitance scheme. For this capacitive sensor, a plate is formed on the uppermost layer of LSI interconnections, and a passivation film is formed thereon.
When a fingertip comes into contact with this sensor, the skin surface functions as a second plate which is spaced apart by an insulating layer formed by air. Sensing is done on the basis of the distance difference between the skin surface and the plate, thereby detecting the fingerprint. In this technique, a reference plate is arranged near the plate on the uppermost layer, and the difference from this reference plate is used for actual sensing. As characteristic features of this structure, no special interface is required, and the size can be reduced, unlike the conventional optical scheme.
In principle, the fingerprint sensor has a sensor electrode formed on a semiconductor substrate and a passivation film formed on the sensor electrode, in which the capacitance between the skin and the sensor is detected through the passivation film to detect a small three-dimensional structure.
The conventional capacitive fingerprint sensor will be briefly described with reference to the accompanying drawings. This capacitive sensor has a structure shown in FIG. 10. An interconnection 403 is formed via a lower insulating film 402 on a semiconductor substrate 401 having LSIs formed thereon, and an interlevel insulator 404 is formed thereon.
Sensor electrodes 406 each having, e.g., a rectangular planar shape are formed on the interlevel insulator 404. The sensor electrode 406 is connected to the interconnection 403 through a plug 405 in the through hole formed in the interlevel insulator 404. A passivation film 407 is formed on the interlevel insulator 404 to cover the sensor electrodes 406, thereby forming a sensor element. As shown in FIG. 11, a plurality of sensor elements are two-dimensionally arrayed while preventing the sensor electrodes 406 of adjacent sensor elements from coming into contact with each other.
The operation of the capacitive sensor will be described next. To detect a fingerprint, a finger whose fingerprint is to be detected comes into contact with the passivation film 407 first. As the finger comes into contact, the skin in contact with the passivation film 407 on the sensor electrode 406 functions as an electrode, so a capacitance is formed between the skin and the sensor electrode 406. This capacitance is detected through the interconnection 403. The fingerprint at the fingertip is formed by the three-dimensional pattern of the skin. Hence, when the fingertip is brought into contact with the passivation film 407, the distance between the sensor electrode 406 and the skin serving as an electrode changes between the ridge portion and the valley portion of the skin surface. This difference in distance is detected as the difference in capacitance. Hence, the three-dimensional pattern on the skin surface can be obtained by detecting the distribution of capacitance that changes between the sensor electrodes. Thus, the small three-dimensional pattern on the skin can be sensed by this capacitive sensor.
Such a capacitive fingerprint sensor requires no special interface and enables size reduction, unlike the conventional optical sensor.
This capacitive sensor can be integrally mounted on an integrated circuit (LSI) chip which integrates the following sections. More specifically, the above-described capacitive sensor can be mounted on an integrated circuit chip which integrates a capacitance detection circuit for detecting the capacitance of the sensor electrode 406, a processing circuit for receiving and processing the output from the capacitance detection circuit, a storage circuit storing fingerprint data for collation, and a comparison/collation circuit for comparing and collating the fingerprint data in the storage circuit with a fingerprint detected by the capacitance detection circuit and processed by the processing circuit. When these units are formed on one integrated circuit chip, information can hardly be altered in data transfer between the units, and security performance can be improved.
A capacitance detection sensor using such an LSI technology is described in, e.g., xe2x80x9cISSCC DIGEST OF TECHNICAL PAPERSxe2x80x9d FEBRUARY 1998 pp. 284-285.
FIG. 12 shows a conventional capacitance detection circuit for detecting an electrostatic capacitance formed between finger skin and an electrode to detect the three-dimensional pattern on the skin surface. Referring to FIG. 12, a detection element 50 outputs, as a voltage signal, a value Cf of electrostatic capacitance formed between the sensor electrode 406 and a surface 400 of a finger in contact. A capacitance detection circuit 500 comprises a signal generation circuit 510 and output circuit 520. The sensor electrode 406 of the detection element 50 is connected to the input side of a current source 511 of a current I through an NMOS transistor Q2. A node N1 between the sensor electrode 406 and the transistor Q2 is connected to the input side of the output circuit 520. A power supply voltage VDD is applied to the node N1 through a PMOS transistor Q1. The node N1 has a parasitic capacitance Cp0. Signals {overscore (PRE)} and RE are supplied to the gate terminals of the transistors Q1 and Q2, respectively.
The current source 511 and transistor Q2 constitute the signal generation circuit 510, and an NMOS transistor Q3 and bias resistance Ra constitute the output circuit 520.
The operation of the capacitance detection circuit 500 shown in FIG. 12 will be described.
First, the signal {overscore (PRE)} of high level (VDD) is supplied to the gate terminal of the transistor Q1 while the signal RE of low level (GND) is supplied to the gate terminal of the transistor Q2. Hence, the transistors Q1 and Q2 are not ON.
In this state, when the signal {overscore (PRE)} changes from high level to low level, the transistor Q1 is turned on. Since the transistor Q2 is kept OFF, the potential at the node N1 is precharged to VDD.
After completion of precharge, the signal {overscore (PRE)} changes to high level, and simultaneously, the signal RE changes to high level. The transistor Q1 is turned off, and the transistor Q2 is turned on. Charges stored at the node N1 are removed by the current source 511. As a result, the potential at the node N1 drops.
Let xcex94t be the period in which the signal RE is kept at high level. A potential drop xcex94V at the node N1 after the elapse of period xcex94t is given by
xcex94V=Ixcex94t/(Cf+Cp0)xe2x80x83xe2x80x83(1)
where Cf is the electrostatic capacitance value.
Since the current I of the current source 511, the period xcex94t, and the parasitic capacitance Cp0 have predetermined values, the potential drop xcex94V is determined by the electrostatic capacitance value Cf. The capacitance value Cf is determined by the distance between the sensor electrode 406 and the finger surface 400 and therefore changes depending on the three-dimensional pattern on the skin surface. This potential drop xcex94V is supplied to the output circuit 520 as an input signal. The output circuit 520 receives the potential drop xcex94V and outputs a signal that reflects the three-dimensional pattern on the skin surface.
However, the above-described capacitive sensor uses the finger skin as an electrode. For this reason, if a finger with static electricity comes into contact with the sensor, the LSI integrated with the capacitive sensor readily electrostatically break due to this static electricity, resulting in degradation in reliability.
More specifically, a MOS transistor of an LSI normally has characteristics representing that a signal is highly sensitively output in response to a signal input to the gate terminal. For this reason, in the conventional capacitance detection circuit 500, the gate terminal of the MOS transistor Q3 of the output circuit 520 is directly connected to the node N1 connected to the sensor electrode 406, thereby highly sensitively detecting the small signal change xcex94V at the node and outputting it.
However, the gate oxide film of the MOS transistor is as thin as 10 nm and has a breakdown voltage of about 100 V. If a voltage higher than this breakdown voltage is input to the gate terminal, the gate oxide film breaks to make the MOS transistor inoperable. For this reason, in recognizing a surface shape such as a three-dimensional pattern on a finger by the conventional capacitance detection circuit shown in FIG. 12, if the target recognition object such as a finger has static electricity, the static electricity more than 1,000 V reaches the gate terminal of the MOS transistor Q3 in the output circuit 520 through the sensor electrode 406. Consequently, the transistor Q3 breaks to degrade the reliability.
It is an object of the present invention to improve the reliability of a surface shape recognition apparatus for recognizing a small surface shape such as a three-dimensional pattern on a finger skin surface using a capacitive sensor.
In order to achieve the above object, according to the present invention, there is provided a surface shape recognition apparatus comprising a plurality of sensor electrodes formed on an interlevel insulator on a substrate and insulated from each other, a passivation film formed on the interlevel insulator to cover an upper surface and side surface of each of the sensor electrodes, the passivation film being formed from a dielectric material, a capacitance detection circuit for, when a target recognition object comes into contact with a surface of the passivation film, detecting an electrostatic capacitance formed between the sensor electrode and a surface of the target recognition object opposing the sensor electrode, and static electricity avoiding means for passing static electricity on the surface of the passivation film.